OBJECTIVES 
The synthetic peptide T11F, with sequence identical to a fragment of the constant region of human IgM, and most of its substituted alanine derivatives proved to possess a significant in vitro candidacidal activity (EC50 values in the micromolar range). In this study, the therapeutic efficacy of T11F and two selected derivatives, D5A, the most active in vitro, and F11A, characterized by a different conformation, was investigated in a Galleria mellonella infection model by Candida albicans. 
METHODS 
Last instar G. mellonella larvae (16 larvae/group) were used for this study. For toxicity experiments, larvae were inoculated directly into the hemocoel, via the last left pro-leg, with the selected peptides (15 µmol/kg). Control groups consisted of larvae untouched or inoculated with saline. Potential therapeutic activity of selected peptides was then evaluated by inoculating C. albicans SC5314 suspension (5×105 cells/larva); 30 min after Candida challenge, larvae were injected via the last right pro-leg with the selected peptides (10 µmol/kg) or saline (control). Larvae were then transferred into clean Petri dishes, incubated at 37 °C in the dark for 9 days, and scored daily for survival. Survival curves of peptide-treated and control animals were compared by the Mantel-Cox log-rank test. A value of P<0.05 was considered significant. Peptide modulation of host immunity to C. albicans infection was determined using hemocyte analysis, by hemocytometer count and cytospin, and larval histology.
RESULTS 
All the tested peptides showed lack of toxicity in this experimental model, with survival curves similar to control group (saline-injected). A single injection of F11A derivative, but not D5A and T11F, in larvae infected with C. albicans SC5314, led to a significant increase in survival, in comparison with infected animals treated with saline. Hemocytes analyses and histology highlighted a different immune stimulation by the studied peptides. In particular, F11A was the most active in eliciting nodule formation and melanization and in fat body activation, leading to a better control of yeast growth. Indeed, F11A-treated larvae showed stronger hemocytes recruitment at tissue level and no yeast hyphal forms were noticed compared with D5A- and T11F-treated groups.
CONCLUSIONS
Overall, the different therapeutic effects observed in vivo against C. albicans systemic infection allow for hypothesizing a critical role in immune system activation. Although all peptides displayed similar activity in vitro, only F11A was efficient in increasing larvae survival, by promoting a protective immune response characterized mainly by granulocytes and plasmatocytes. Our data suggest for F11A a double role, targeting simultaneously the fungus and the immune system, which results in pathogen clearance.

1 / Therapeutic effects of an antibody-derived peptide and its alanine scanning derivatives in a Galleria mellonella model of systemic candidiasis